Not Applicable.
Not Applicable.
In pipeline operations and other industrial applications, flow meters must be capable of accurately measuring the flow rate of gases or liquids moving through piping or tubing systems. In natural gas pipelines, for example, these flow rate measurements may be relied upon for custody transfer, leak detection, control, or for other indications.
For custody transfer operations, the meter is the point where custody transfer occurs, such as when gas is delivered into or out of a pipeline system through the meter as it measures the passing flow rate. By accurately measuring the flow rate for a given time period, the volume of gas that passes through the meter can be determined, and a custody transfer volume ticket can then be prepared. The pipeline transportation fee is based on the volume of product moved through the system, ie. the custody transfer volume. Thus, a custody transfer metering system is commonly referred to in the pipeline industry as the xe2x80x9ccash register,xe2x80x9d and pipeline operators take great care to maintain its measurement accuracy.
Concentric orifice meters are one type of flow meter used to measure the flow of fluid through a pipeline. More recently, ultrasonic flow meters have been introduced that use ultrasonic signals to detect the flow velocity of fluid (i.e. liquid or gas) moving through a pipeline. All flow meters, however, must accurately measure the speed of the flowing fluid.
Often the fluid stream is a gas containing a certain amount, or percent fraction, of liquid. This is referred to wet gas, or more generally, as two-phase flow. The liquid in the gas often travels in either a mist flow or a stratified flow, or as a combination of these two. In mist flow, tiny droplets of liquid disperse through and are carried along by the gaseous flow. Thus, the droplets travel at about the same speed as the gas. In stratified flow, the liquid travels in a flow or river most often. at the bottom of the pipe. In stratified flow, the liquid typically travels at a different, and usually slower, speed than the gas stream. Although not as common, the two-phase flow may be predominantly liquid with a percent fraction of gas traveling in a stratified flow at the top of the pipeline.
To determine the amount of gas and liquid flowing in the pipeline, it is necessary to determine the relative areas occupied by the gas and liquid flowing through the pipeline, the speed of the gas stream, and the speed of the liquid flow. It would also be helpful to know the composition of the gas and liquid portions. For example, if the gas contains xe2x80x9cnatural gas liquidsxe2x80x9d or condensates, a seller of gas deserves extra compensation for this energy-rich liquid. Unfortunately, however, flow meters have historically not been very good at measuring the flow rate (i.e. volume or mass flow) of both a liquid fraction and a gas fraction for wet gas flow.
Various measurements of an ultrasonic flow meter have been found to correlate to the amount of liquid travelling as a mist flow, as taught in U.S. Pat. No. 6,151,158, xe2x80x9cUltrasonic Fraction and Flow Apparatus and Method,xe2x80x9d Ser. No. 08/613,478, hereby incorporated by reference for all purposes without exception. However, this patent does not describe how to determine the amount of stratified flow in the pipeline, its velocity, or its composition.
A flow meter is therefore needed that is capable of measuring the amount of stratified fluid flow in a pipeline. Ideally, this flow meter could determine the flow velocity of the stratified flow as well. It would also be desirable if this flow meter could determine the composition of the stratified flow. In a perfect world, this flow meter would also measure the flow velocity of the gas flow, the area of the pipeline occupied by the gas flow, and the composition of the gas flow. It would further be desirable if this flow meter could also detect the presence of a mist flow.
A disclosed embodiment of the present invention features an ultrasonic flow meter suitable to to carry a two-phase flow having a stratified flow portion. A first path, corresponding to a first ultrasonic transducer, travels wholly through one phase of the two-phase flow. A second path, corresponding to a second ultrasonic transducer, traverses through one phase of the two-phase flow, but reflects or bounces from the interface between the gas and liquid flows. A third path, corresponding to a third ultrasonic transducer, traverses through the other phase of the two-phase flow, and also reflects or bounces from the interface between the gas and liquid flows. Each of these first, second, and third transducers generally correspond to another transducer, making at least three pairs of transducers in the flow meter.
Based on this configuration, an associated processor calculates flow velocity and speed of sound for each phase of the two-phase flow. The level of the stratified flow is also determined, as well as the average density or molecular weight of each of the phases. This provides an indication of the composition of each of these phases.
Thus, the invention comprises a combination of features and advantages which enable it to overcome various problems of prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.